Since diamond has many excellent characteristics such as high hardness, high thermal conductivity, high light transmittance and wide band gap, it has been widely utilized as a material for some kinds of tools, optical components, semiconductors and electronic devices and importance of diamond is expected to increase in the future. Diamond which was naturally produced served industrial use in the past but now architecturally synthesized diamond is supplied for most of industrial use. Single crystalline diamond is synthesized under high pressure over tens of thousands of atmospheric pressure in the current industry. Extremely high pressure vessels that provide such high pressure are very expensive and their size has to be limited, then single crystal synthesis of a larger scaled single crystal by high temperature-high pressure method is restricted. Ib type diamond, which displays yellow due to nitrogen included as an impurity, is synthesized up to 1 cm diameter by high temperature-high pressure method and sold, but, that size is regarded as an upper limit. Moreover, IIa type diamond, which is colorless and clear, is limited to more smaller size of under several millimeter diameter except for natural products.
Chemical vapor deposition is an established method for diamond synthesis as well as high pressure method. A comparatively large area of several to ten centimeter is architecturally produced by this method and usually it is polycrystalline film. But, single crystalline diamond is necessary to use for diamond applications such as high precision tools, optical components and semiconductors, because especially smooth surface is needed among diamond uses. Accordingly, a method to obtain single crystalline diamond epitaxially grown by chemical vapor deposition has been studied.
Epitaxial growth is generally classified into two, one is homoepitaxial growth to raise the growing substance on a substrate of the same component, and heteroepitaxial growth to grow on a substrate of a different component. Heteroepitaxial growth of cubic boron nitride (cBN), silicon carbide, silicon, nickel, cobalt or the like has been reported (see Japanese Patent Publication Nos. S63-224225, H2-233591 and H4-132687). However, single crystal having good film quality has not obtained by heteroepitaxial growth, then heteroepitaxial growth is regarded as a predominant for single crystal synthesis. With homoepitaxial growth, IIa single crystalline diamond which is larger than the IIa diamond obtained under high pressure is obtained by epitaxially growing high purity diamond from gas phase on a Ib diamond produced by high pressure synthesis. It was reported that, by using a plurality of diamond substrates whose crystal orientations were the same or diamond particles, diamond having only a small inclined grain boundary was obtained by growing unified diamond thereon (see Japanese Patent Publication No. H3-75298).